Ozone risk assessment is affected by nutrient availability: Evidence from a simulation experiment under free air controlled exposure (FACE).

Assessing ozone (O) risk to vegetation is crucial for informing policy making. Soil nitrogen (N) and phosphorus (P) availability could change stomatal conductance which is the main driver of O uptake into a leaf. In addition, the availability of N and P could influence photosynthesis and growth. We thus postulated that the sensitivity of plants to O may be changed by the levels of N and P in the soil. In this study, a sensitive poplar clone (Oxford) was subject to two N levels (N0, 0 kg N ha; N80, 80 kg N ha), three P levels (P0, 0 kg P ha; P40, 40 kg P ha; P80, 80 kg P ha) and three levels of O exposure (ambient concentration, AA; 1.5 × AA; 2.0 × AA) for a whole growing season in an O free air controlled exposure (FACE) facility. Flux-based (POD) and exposure-based (W126 and AOT40) dose-response relationships were fitted and critical levels (CLs) were estimated for a 5% decrease of total annual biomass. It was found that N and P availability modified the dose-response relationships of biomass responses to O. Overall, the N supply decreased the O CLs i.e. increased the sensitivity of poplar to O. Phosphorus alleviated the O-caused biomass loss and increased the CL. However, such mitigation effects of P were found only in low N and not in high N conditions. In each nutritional treatment, similar performance was found between flux-based and exposure-based indices. However, the flux-based approach was superior, as compared to exposure indices, to explain the biomass reduction when all nutritional treatments were pooled together. The best O metric for risk assessments was POD, with 4.6 mmol m POD as a suitable CL for Oxford poplars grown under various soil N and P conditions.